This invention relates to high frequency oscillators or inverters, and more particularly relates to oscillator or inverter circuits which develop a relatively high frequency magnetic flux and produce a transformer action which may be used to effect induction heating of a cooking utensil and which may be used for isolation and to develop power for resistance welding, or the like.
In the following description the electric circuits of the invention will be described principally with reference to use thereof for induction cooking wherein a high frequency magnetic flux generated in an inductive work coil, i.e. an inductor which generates a magnetic output signal in response to a supplied current, in an inverter circuit is applied, for example, to a magnetically responsive steel cooking utensil or the like to heat the same for cooking food therein; however, it is to be clearly understood that the described inverter circuits also may be used in other applications where high frequency signals are to be developed from relatively low frequency AC and/or DC input power and, particularly, where such high frequency signals are converted to magnetic flux signals that provide a transformer action for heating a magnetically responsive material. Moreover, in electric welding processes or the like, which require large amounts of power transmitted through a large isolation transformer when relatively low frequency signals are used, the inverters of the invention provide an advantage in the generation of high frequency signals that may be efficiently coupled through a relatively small isolation transformer to provide power for welding or the like, thus reducing the cost of such equipment.
Although the broad principle of induction heating has been known, present interest in safety, reliability and efficiency in heating and, particularly, in cooking has spurred development of modern induction heating technology. In induction heating it is desirable to avoid the generation of radio frequency interference when generating the required high frequency signals, and it is also desirable to be able to generate the high frequency signals using conveniently available relatively low cost circuit elements, such as SCR switches or the like, as opposed to those circuit elements that may require still further technological development with the appurtenant increased costs thereof. It is also desirable to time share circuit components for further system cost reduction as well as to conserve power in the inverter while being able to energize more than one inductive work coil, each of which may represent a burner or heating element of a range, oven and/or other cooking appliance.
Moreover, the frequency of the magnetic flux generated in an inductive work coil of an induction heating inverter circuit should be as high as possible, such as, for example, in the 20 KHz to 50 KHz range, and preferably closer to the upper end of the range. It has been found in induction cooking that the higher the frequency of the magnetic flux signals, the more efficient is the heating of the pan since a more concentrated heating closer to the skin portion of the pan is achieved compared to the deeper and less concentrated heating that is obtained using lower frequency signals. Also the efficiency of power transferred in a transformer action, wherein magnetic flux signals are developed in one circuit component and are transferred across an air gap or through a core to another circuit component, such as a magnetically responsive cooking utensil or the like, which forms effectively a short circuited single turn secondary, or a transformer secondary, increases with an increase in the signal frequency. It is, of course, also desirable to energize independently a plurality of inductive work coils, each of which may be a respective heating element of a range, oven and/or other cooking appliance.
Several recent patents have been concerned with induction heating. In U.S. Pat. No. 3,814,888 a high frequency electric signal is applied to an inductive work coil to generate a high frequency magnetic flux, which is coupled across an effective air gap to a magnetically responsive cooking utensil for heating the same. A basic inverter circuit which generates a high frequency AC signal from a constant input DC voltage power supply is disclosed in U.S. Pat. No. 3,047,789, and a modification of that inverter circuit is disclosed in U.S. Pat. No. 3,404,327, wherein center tapped or split inductance elements are used. In U.S. Pat. No. 3,637,970 a voltage fed oscillator for induction heating is disclosed, and an inductive work coil for cooking use is shown in U.S. Pat. No. 3,256,417.